The present invention relates to aviation electronics (avionics). More particularly, the present invention is related to a method and apparatus for displaying a preselector altitude (PSA) value and for adaptively changing a manner in which the PSA value is changed.
Complex aircraft of today require electronic instruments which provide a large amount of information displayed to a pilot of the aircraft. One type of display unit for providing information to a pilot is a Primary Flight Display (PFD). PFD""s display flight critical information to the pilot on a relatively compact display screen. The information displayed often includes an air speed indicator, an altimeter, a vertical speed indicator, an artificial horizon indicator and other appropriate display information. Another piece of information that is typically displayed on a PFD is a preselect altitude (PSA) value or setting. The PSA value, which is typically displayed on the PFD above the altimeter, is provided to the auto pilot. The auto pilot is programmed to level off and maintain the aircraft at the altitude indicated by the PSA value. The PSA value is normally set by the pilot in one hundred foot increments.
Another feature which is typically displayed on a PFD is the BARO MIN reference value. The BARO MIN reference is a displayed altitude at which, if the pilot cannot visibly see the runway, the landing approach is to be aborted. The BARO MIN reference is determined by the Federal Aviation Authority (FAA), and varies in increments of ten feet. Since the PSA value can typically be changed in one hundred foot increments, while the BARO MIN reference value is set in ten foot increments, it is frequently the case that the PSA value must be set at an altitude which is between ten feet and ninety feet above the BARO MIN reference value. Consequently, the pilot of the aircraft may have to abort a particular landing approach once the aircraft reaches the PSA value altitude, without actually reaching the BARO MIN altitude. In some of these instances, if the aircraft were allowed to descend to the BARO MIN altitude, the runway would have been visible to the pilot and it would not have been necessary to abort the landing approach.
A method of changing a preselect altitude value includes receiving an input from a pilot. In response, the preselect altitude value is changed by a first amount from a current preselect altitude value toward a minimum altitude value. The first amount that the preselect altitude value is changed by is dependent upon a proximity of the current preselect altitude value to the minimum altitude value such that if a difference between the current preselect altitude value and the minimum altitude value is greater than a first predetermined interval, the first amount is equal to the first predetermined interval. If the difference between the current preselect altitude value and the minimum altitude value is less than the first predetermined interval, then the first amount is less than the first interval, for example resulting in the preselect altitude value being set directly to the minimum altitude value.